Suppressing Dendritic Lithium Formation Using Porous Media in

Mar 1, 2018 - (1−3) However, the recent development of rechargeable Li metal batteries (LiMBs) has faced regular setbacks due to the growth of Li de...
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Suppressing Dendritic Lithium Formation Using Porous Media in Lithium Metal-Based Batteries Nan Li,† Wenfei Wei,† Keyu Xie,*,† Jinwang Tan,§,# Lin Zhang,∥ Xiaodong Luo,⊥ Kai Yuan,† Qiang Song,† Hejun Li,† Chao Shen,† Emily M. Ryan,§ Ling Liu,∥ and Bingqing Wei*,†,‡ †

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi’an 710072, China ‡ Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States § Department of Mechanical Engineering, Boston University, 110 Cummington Mall, Boston, Massachusetts 02215, United States ∥ Department of Mechanical and Aerospace Engineering, Utah State University, Logan, Utah 84322, United States ⊥ College of Metallurgical and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401311, China # College of Mechatronics and Control Engineering, Shenzhen University, Shenzhen 518060, China S Supporting Information *

ABSTRACT: Because of its ultrahigh specific capacity, lithium metal holds great promise for revolutionizing current rechargeable battery technologies. Nevertheless, the unavoidable formation of dendritic Li, as well as the resulting safety hazards and poor cycling stability, have significantly hindered its practical applications. A mainstream strategy to solve this problem is introducing porous media, such as solid electrolytes, modified separators, or artificial protection layers, to block Li dendrite penetration. However, the scientific foundation of this strategy has not yet been elucidated. Herein, using experiments and simulation we analyze the role of the porous media in suppressing dendritic Li growth and probe the underlying fundamental mechanisms. It is found that the tortuous pores of the porous media, which drastically reduce the local flux of Li+ moving toward the anode and effectively extend the physical path of dendrite growth, are the key to achieving the nondendritic Li growth. On the basis of the theoretical exploration, we synthesize a novel porous silicon nitride submicron-wire membrane and incorporate it in both half-cell and full-cell configurations. The operation time of the battery cells is significantly extended without a short circuit. The findings lay the foundation to use a porous medium for achieving nondendritic Li growth in Li metal-based batteries. KEYWORDS: Lithium anode, dendrite, porous media, fundamental mechanisms stability of the SEI. However, most of these modified SEI films are still not stable enough to survive the mechanical deformation accompanying repeated Li deposition and stripping;14 thus, the safety and long-term stability of the Li anode cannot be guaranteed. Recently, introducing submicron-/nanoporous media (e.g., solid electrolytes,15−17 modified separators,18−20 and artificial protection layers21−25) in LiMBs has emerged as a promising alternative strategy to solve the notorious dendrite growth problem for its versatility, simpleness, and high efficiency. However, an in-depth fundamental understanding of these experiments is lacking,20,26 which has hindered the continued development of these strategies to revive LiMBs further. Because the solid electrolyte, modified separator, and artificial protection layer all have submicron-/nanoporous structures, their dendrite suppression performance is hypo-

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s the most electropositive metal (−3.040 V vs the standard hydrogen electrode) that has an ultrahigh theoretical specific capacity of 3860 mAh g−1, lithium (Li) holds great promise for revolutionizing current rechargeable battery technologies.1−3 However, the recent development of rechargeable Li metal batteries (LiMBs) has faced regular setbacks due to the growth of Li dendrites, a phenomenon that is hard to control yet substantially compromises the performance of LiMBs, leading to low Coulombic efficiency, poor cycle stability, and even short-circuiting-related safety hazards.4,5 To conquer this challenge and commercialize high-energy LiMBs, a new round of “gold rush” has been launched in related academic and industrial fields,1,6 and several strategies have been proposed to suppress dendrite growth. For example, an interface engineering approach aiming to obtain a stable solidelectrolyte interface (SEI) film on the Li surface has been developed by tuning the compositions of electrolytes and additives. Liquid electrolytes with different Li salts (e.g., LiTFSI, LiFSI, LiBOB, or their mixtures),7−9 solvents (e.g., carbonates, ethers, or ionic liquids),10,11 and additives (e.g., Cs+, LiF, LiNO3, Li2S8)12,13 have been demonstrated to improve the © XXXX American Chemical Society

Received: January 13, 2018 Revised: February 16, 2018 Published: March 1, 2018 A

DOI: 10.1021/acs.nanolett.8b00183 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

impeded by the porous membrane due to the reduced local mass transport of Li+ from the bulk phase to the anode surface and the diffusion difficulty rendered by the tortuosity of the porous medium, as discussed below. To reveal reactive transport mechanisms underpinning the phenomenon of dendrite suppression, two pointwise quantities (i.e., the concentration of the Li+ ion, C, and the Li+ transport flux, − D∇C) are investigated (Figure 1e,f). Li+ concentration in the control group (without the membrane) is linearly distributed in the y-direction with no variations in the xdirection, making it equivalent to a one-dimensional solution. However, when the porous membrane is introduced, Li+ distribution becomes highly heterogeneous, featuring prominent variations along the tortuous pores. In fact, the physical suppression effect originates from the geometrically restrictive pores that force dendrites to grow tortuously, which effectively reduces the speed of dendrite growth toward the separator. Meanwhile, the existence of the porous membrane also hinders local mass transport of Li+ from the bulk phase to the anode surface. As shown in Figure 1f, while the Li+ ions always attempt to move to the “hot spots”29,30 of generating dendrites, the porous medium reduces the local Li+ transport flux, leading to reduced Li+ deposition near the anode surface. In summary, dendrite formation and growth can be efficiently suppressed in the presence of a porous medium due to the locally reduced Li+ transport flux as well as the physical suppression effect caused by pore tortuosity. To further confirm these results, another porous membrane was modeled with a decreased porosity of 0.34 (10% less) and a reduced tortuosity. As expected, more and longer dendrites are formed in the system (Supporting Figure S2), indicating reduced suppression due to the less porous and less tortuous membrane, which has less effect on the local Li+ transport flux. Notably, despite the changes in geometry and efficiency dendrite suppression associated with the porous membranes is preserved. To verify the simulation findings, a free-standing α-Si3N4 submicron-wire membrane has been synthesized and introduced as a new porous medium for dendrite control and other functional applications.31 To suppress dendrite formation and growth, a porous medium should hold good mechanical properties, high stability in electrolytes, and most importantly, excellent wettability with the electrolytes. Prepared following an approach illustrated in Supporting Figure S3, the as-synthesized Si3N4 membrane has good mechanical properties (Supporting Figure S4) and can be rolled on a graphite paper (Figure 2a). On the basis of TEM images (Figure 2b,c), the Si3N4 submicron-wires are single crystals, and all possess a perfect crystal structure.32 The result is also verified by XRD, which reveals that the submicron-wires are pure α-Si3N4 crystals (Figure 2g). Additionally, these submicron-wires, which are more than several millimeters long (Supporting Figure S5), show relatively uniform morphology with a typical hole diameter of 1000 nm; the two elements, silicon and nitrogen, are uniformly distributed (Figure 2d−f). The pore size distribution of α-Si3N4 submicron-wire membrane indicates that the pores are mainly macropores with few micropores and mesopores (Supporting Figure S6). Meanwhile, the membrane also features abundant polar covalent bonds on its surfaces to serve as anchoring points for binding with Li+ ions.25 On the one hand, Fourier transform infrared spectrometry (Figure 2h) shows vibrational bands at 840, 885, and 1053 cm−1, which correspond to the stretching vibration mode of Si−N,33 a polar

thetically associated with the shared structural feature, that is, a porous medium. As a proof-of-principle, we fabricated and incorporated a novel porous silicon nitride (α-Si 3 N4 ) submicron-wire membrane in symmetric Li/Li cells, which have shown no apparent dendrite growth after cycling for more than 3000 h. Given the various possible combinations of components and materials in batteries, a systematic investigation around the effects of porous geometry could provide guidance for achieving Li and other metal-based (e.g., Na, K, Zn, and Al) batteries without critical dendritic metal growth. To probe fundamental mechanisms underpinning the dendritic Li suppression behavior with porous media, numerical simulations have been performed using a mesoscale particle based Lagrangian method.27,28 A porous medium (a thin layer of the porous membrane with randomly oriented fibers, Figure 1a) was modeled and introduced to cover the entire surface of

Figure 1. Simulation of Li growth with or without the porous membrane. (a−d) Dendrite growth simulation results at the reaction rate of K = 50 μm s−1 (a,b) and 5 μm s−1 (c,d) (gray, a liquid electrolyte; black, anode surface and porous membrane structure; white, dendrites). (e,f) Li+ concentration overlaid with the mass transport flux near the anode surface with the fiber membrane (e) and without the fiber membrane (f) for K = 5 μm s−1. All results are extracted when t = 1500 s. Note that dendrite deposition is disabled in panels e and f to eliminate the growing reactive boundary of the dendrite so that the effects of pore structure on mass transport can be seen.

the lithium anode. Values and units for all parameters used in the simulations are listed in Table S1. On the basis of lowerscale molecular dynamics simulations (Supporting Figure S1), the effects of migration and convection on solute dispersion were neglected in the mesoscale simulation. After 1500 s of charging at a high Li+ reaction rate of K = 50 μm s−1, the longest and most critical dendrite formed inside the torturous pores is only 3 μm in trunk length (Figure 1a), 75% shorter than that formed in the absence of the porous membrane (Figure 1b). The results demonstrate a strong effect of the porous layer impeding dendrite growth. Meanwhile, the porous structure also suppresses dendrite initiation as evidenced by the fewer number of dendrite growth sites. Only four apparent dendrites are generated on the anode with the porous membrane, whereas more than 20 are formed without the porous membrane (Figure 1a,b). Similar results are obtained when the charging (reaction) rate is lowered to K = 5 μm s−1 (Figure 1c,d). These simulations show that dendrite formation (i.e., number of dendrites) and growth (i.e., length) are both B

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Figure 2. Morphology and structure of the α-Si3N4 submicron-wire membrane. (a) Photograph of the partly rolled α-Si3N4 membrane on the graphite paper. (b) TEM image of a single α-Si3N4 submicron-wire, and (c) its high-resolution image (inset is the electron diffraction pattern of the α-Si3N4 submicron-wire). (d) FESEM of the α-Si3N4 membrane and the corresponding element mapping of (e) Si and (f) N. (g) XRD pattern and (h) FTIR of the α-Si3N4 submicron-wires. (i) Contact angles of the electrolyte on the commercial polymer separator and the α-Si3N4 submicronwire membrane.

Figure 3. Li deposition on different substrates. Schematic illustrations of Li deposition (a) on the Cu current collector within the α-Si3N4 membrane and (b) bare Cu current collector. (c,g) The top-view and cross-sectional SEM images of Li deposited within the α-Si3N4 membrane after 100 cycles at a current density of 1.0 mA cm−2. (d,h) Magnified views of panels c and g, respectively. (e,i) The top-view and cross-sectional SEM images of the Li metal anode on the bare Cu current collector after 100 cycles at a current density of 1.0 mA cm−2. (f,j) Magnified views of panels e and i, respectively.

redox peak can be found from the CV curve of the α-Si3N4|Li cell within a wide potential range of 0−4.5 V (vs Li/Li+, Supporting Figure S7). Collectively, all these properties make the submicron-wire membrane an excellent three-dimensional (3D) porous network that can effectively and durably entrap

covalent bond that can interact strongly with Li+ ions. On the other hand, the contact angle of the electrolyte is about 57°on the commercial polymer separator and almost 0° on the αSi3N4 membrane (Figure 2i), indicating excellent wettability of the membrane and enhanced affinity with the electrolyte. In addition, the as-synthesized Si3N4 is electrochemically inert; no C

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Figure 4. Electrochemical characterization of the bare Cu current collector and the α-Si3N4-membrane-covered Cu current collector. (a) Cycling performances of the α-Si3N4-membrane-covered Cu foil and the bare Cu foil at 0.5, 1.0, and 2.0 mA cm−2, respectively. The deposition capacity of Li is fixed at 1 mAh cm−2. (b) The polarization of plating/stripping for the α-Si3N4-membrane-covered Cu foil and the bared Cu foil at different current densities at the 15th cycle, respectively. Inset in b is an expanded view of the bottom plot. (c) Voltage−time curves of Li deposition/stripping in the symmetrical Li|Li cell (top) and the Li-α-Si3N4|α-Si3N4−Li cell (bottom). Inset: Magnified view of the voltage−time curve of the Li|Li cell. The amount of plated Li is 1.0 mAh cm−2, and the current density is 1.0 mA cm−2 in each cycle.

Li+ ions for inhibiting dendrite formation23,25 and other functional applications. To verify the functionality of the 3D porous α-Si3N4 submicron-wire membrane for dendrite suppression, a αSi3N4 membrane is introduced on the top of a Cu foil current collector (Figure 3a). For comparison, a planar bare Cu foil is also introduced as the current collector. The electrolyte used here is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a cosolvent of 1,3-dioxolane (DOL), and 1,2-dimethoxyethane (DME) with 2% LiNO3. In principle, after repeated Li deposition and stripping, the planar bare Cu foil should be covered with dense, long Li dendrites (Figure 3b), which drastically increase safety risks.34,35 Meanwhile, the SEI film is repeatedly broken and regenerated during the cycling, leading to continuous consumption of Li in the cell,36 low Coulombic efficiencies, and poor cycling stability. Our experiments show consistent results. Figure 3e,f,i,j shows that a crowd of apical and tree-like Li dendrites has appeared and protruded on the bare Cu foil after 100 cycles. Because of the growth of dendrites, the Li layer on the bare Cu foil is extremely loose, and the rough thickness of the Li layer on the bare Cu is about 15 μm (Figure 3i). In addition, the typical length of the dendrites is more than 8 μm with the diameter less than 1 μm. In contrast, dendrite-free Li deposits have been observed within the 3D porous α-Si3N4 membrane, leading to a more compact Li/α-Si3N4 mixture layer. As illustrated in Figure 3c,d, the surface of the Cu foil covered with the α-Si3N4 membrane is remarkably smooth, and no apparent dendrite is observed. The deposited Li has an almost uniform thickness, filling gaps between the submicron-wires (Figure 3g,h). After Li stripping, the 3D porous network of α-Si3N4 membrane, which can

effectively entrap Li, can be retained for inhibiting dendrite formation, as shown in Supporting Figure S8. Coulombic efficiency (CE) is an indicator of long-term cycling stability.35 Herein, CE is defined as the ratio of the amount of Li stripped back to the total amount of Li plated onto the working electrode during each cycle. As shown in Figure 4a, the initial CEs of both the α-Si3N4-membranecovered Cu and the bare Cu foil are 98% and 96% at 0.5 mA cm−2, respectively, indicating that the introduction of the inert α-Si3N4 membrane does not impair the electrochemical reversibility of Li deposition/stripping. However, after 100 cycles the CE of the membrane-covered Cu foil remains more than 90%, while the CE of the bare Cu foil drops to ∼70%. When the current density is further increased up to 1.0 and 2.0 mA cm−2, more apparent CE decay with the bare Cu foil has been observed. The CE of the bare Cu foil dramatically decreases from 94% to 48% after 100 cycles at 1.0 mA cm−2 and from 90% to 22% after 100 cycles at 2.0 mA cm−2, respectively. Furthermore, the CE of the bare Cu foil shows prominent and irregular fluctuations during the long-term Li deposition and stripping cycles, which may be attributed to partial shortcircuits originating from the appearance of Li dendrites,37 as shown in Figure 3c,g. By contrast, the CE of the membranecovered Cu foil remains nearly constant at a high level of ∼90% due to the dendrite-free Li deposition. Similar results can also be found using 1.0 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 in volume) at 1.0 mA cm−2 (Supporting Figure S9). The CE of the bare Cu foil in a carbonate-based electrolyte shows inferior electrochemical performance during the long-term Li deposition and stripping cycles due to the appearance of Li dendrites. In comparison, the D

DOI: 10.1021/acs.nanolett.8b00183 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 5. Electrochemical performance of the LiFePO4| Li battery with or without the α-Si3N4 membrane and the corresponding morphology of the Li anode. (a) Cycling performances and (b) rate capabilities of the LiFePO4|α-Si3N4−Li and LiFePO4|Li full cells consisting of a solution of 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume). The areal mass loading of the LiFePO4 electrodes was ∼4 mg cm−2. (c,g) The top-view and cross-sectional SEM images of the Li anode in the LiFePO4|Li full cell after 350 cycles. (d,h) Magnified views of C and G respectively. (e,i) The top-view and cross-sectional SEM images of the Li anode in LiFePO4|α-Si3N4−Li after 350 cycles. (f,j) Magnified views of panels e and i, respectively.

CE of the α-Si3N4-membrane-covered Cu foil in a carbonatebased electrolyte presents a more stable cycling life with a nearly constant CE of 95%, due to the homogeneous and dendrite-free Li deposition. These results show that the αSi3N4-membrane-covered Cu foil has a more stable cycling performance and a longer cycle life than its bare Cu counterpart in both ester and ether electrolytes. In general, the introduction of a nonconductive interlayer will increase the internal resistance as well as voltage hysteresis of the cell. Surprisingly, all overpotentials of the α-Si3N4membrane-covered electrode during Li plating and stripping at 0.5, 1.0, and 2.0 mA cm−2 are lower than that of the bare Cu foil after 15 cycles (Figure 4b). The voltage hystereses are 14.4, 18.6, and 31.7 mV, respectively, for the membrane-covered Cu foil, as compared to 14.6, 25.7, and 42.9 mV, respectively, for the bare Cu foil (Inset of Figure 4b). The introduction of the porous membrane reduces the local flux of Li+ mass transport, which typically increases the voltage hysteresis of the cells. And thus, the decrease of voltage hysteresis in cells integrating the membrane may be attributed to the more stable SEI film formed on the nondendritic Li surface after cycles.4,38 Similar phenomena can also be found in previous work.18,21,22,25 A symmetric Li-α-Si3N4|α-Si3N4-Li cell, assembled using two identical α-Si3N4-membrane-covered Li metal sheets as the electrodes, was tested to evaluate further the cycling stability of systems incorporating the α-Si3N4 membranes (Figure 4c). A symmetric bare Li|Li cell was also constructed for comparison. A sudden drop in voltage is found for the symmetric Li|Li cell after only 150 h, which is typical short-circuiting behavior due to dendrite growth.37 As for its counterpart (the symmetric Liα-Si3N4|α-Si3N4-Li cell), the cell can continuously operate for more than 3000 h without any sign of a short circuit. Similar behavior can be found using a different electrolyte of 1.0 M

LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume) at 1.0 mA cm−2 (Supporting Figure S10). The voltage begins to increase after 40 h in the symmetrical Li|Li cell, which indicates an internal short circuit due to dendrite growth. However, the voltage−time curve of a symmetrical Liα-Si3N4|α-Si3N4−Li cell shows very stable performance even after 120 h with no sign of a short circuit. The investigation was further extended to full LiMB cells using the α-Si3N4-membrane-covered Li metal as the anode and commercial LiFePO4 as the cathode (i.e., LiFePO4|α-Si3N4− Li). The areal mass loading of the LiFePO4 electrodes was ∼4 mg cm−2. In the initial cycles, both the cells with and without the α-Si3N4 membrane exhibit a similar CE (∼81%) at 1 C. The discharge capacity of LiFePO4|α-Si3N4−Li and LiFePO4|Li cells are also very close (125 mAh g−1 vs 129 mAh g−1), which indicates that the introduction of the chemically and electrochemically inert α-Si3N4 does not negatively affect the full-cell system. Meanwhile, the discharge capacity of the LiFePO4|αSi3N4−Li cell remains at a high level of 126 mAh g−1 after 350 cycles, whereas the LiFePO4|Li cell severely deteriorates after only 150 cycles, mainly due to the massive formation of dendrites (Figure 5c,d). At a low current density of 0.1 C, both cells deliver a close discharge capacity of ∼150 mAh g−1. However, when the current density is increased to 5 C a discharge capacity of 89 mAh g−1 is achieved in the LiFePO4|αSi3N4−Li cell, much higher than that shown for the LiFePO4|Li cell (49 mAh g−1). To further investigate the α-Si3N4membrane-covered Li metal as the anode in practical full LiMB cells, a much higher areal mass loading of the LiFePO4 electrode (∼10 mg cm−2) was also studied (Supporting Figure S11). The result shows good cycle performance and rate capacity in the LiFePO4|α-Si3N4−Li full cell with a high areal mass loading of ∼10 mg cm−2 of the LiFePO4 electrode. To E

DOI: 10.1021/acs.nanolett.8b00183 Nano Lett. XXXX, XXX, XXX−XXX

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understand this remarkable result, electrochemical impedance spectroscopy (EIS) was used to evaluate kinetic features of these two cells before and after cycles (Supporting Figure S12). The charge-transfer resistances of both cells in the fresh condition are very close to each other (Supporting Figure S12a). However, after cycling the charge-transfer resistance of the LiFePO4|Li cell becomes much higher than that of the LiFePO4|α-Si3N4−Li cell (Supporting Figure S12b), indicating that the use of the α-Si3N4 membrane leads to a more stable SEI film and accordingly, faster charge transfer.39 Moreover, the charge/discharge curves of LiFePO4|α-Si3N4−Li and LiFePO4| Li full cells with different LiFePO4 areal mass loading are also invesgated (Supporting Figure S13). It shows no obvious distinction in the charge/discharge platforms in these full cells with or without a α-Si3N4 membrane, indicating that the introduction of the inert α-Si3N4 membrane does not impair the electrochemical performances of full cells. To further verify the suppression of dendritic Li growth facilitated by the submicron-/nanoporous medium in full LiMBs, morphologies of the Li anodes in both full cells were re-evaluated after 350 cycles (Figure 5c−j). The results agree well with that from the half-cell tests (Figure 5c−f). It is noted that for the bare Li anode the whole Li sheet becomes fluffy (Figure 5g) with a mossy Li surface (Figure 5h). This mossy Li surface will further accelerate the formation of Li dendrites and “dead Li”. By contrast, the Li sheet under the α-Si3N4 membrane remains compact with a smooth surface (Figure 5i), implying that Li is uniformly deposited within the porous membrane. Using experiments and simulation, we have elucidated fundamental mechanisms underpinning the suppression of the dendritic Li growth process in LiMBs by incorporating a submicron-/nanoporous medium. Mesoscale simulations reveal that the tortuous pores of the porous media are the key to achieving the nondendritic Li growth. On the one hand, the tortuous pores drastically reduce the local flux of Li+ moving toward the anode; on the other hand, they effectively extend the physical path of dendrite growth. As a proof of principle, a single-crystalline Si3N4 submicron-wire membrane is fabricated and demonstrated to enable nondendritic Li growth in both half-cell and full-cell configurations. Remarkably, the symmetric Li/Li cell coupled with the Si3N4 membrane can be cycled for more than 3000 h without a short circuit; similar excellent performance is also achieved in the full-cell. The strategy directly addresses the biggest challenges for commercializing LiMBs, that is, safety risks and performance deterioration caused by dendrite growth. The study provides enlightenment and guidance to develop a new generation of metal-based batteries that are safer and more durable.

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Ling Liu: 0000-0001-8743-4570 Bingqing Wei: 0000-0002-9416-1731 Author Contributions

K.Y.X. and B.Q.W. conceived and designed this work. N.L., W.F.W., K.Y., and C.S. carried out parts of the material characterization and the electrochemical measurements. Q.S. and H.J.L. participated in the materials synthesis and characterization. J.W.T. and E.M.R. carried out the mesoscale simulation. L.L., L.Z., and X.D.L. carried out molecular dynamics simulation. K.Y.X., L.L., and B.Q.W. participated in the analysis and discussions of the results, as well as preparation of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of this work by the National Natural Science Foundation of China (51674202, 51402236, 51472204, and 51521061), the Fundamental Research Funds for the Central Universities (G2016KY0307 and 3102017HQZZ007), the Key R&D Program of Shaanxi (2017ZDCXL-GY-08-03), the TOP International University Visiting Program for Outstanding Young Scholars of Northwestern Polytechnical University, and the Natural Science Foundation of SZU (Grant 2017038).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00183.

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DOI: 10.1021/acs.nanolett.8b00183 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.8b00183 Nano Lett. XXXX, XXX, XXX−XXX